I think the main point of confusion in your question is the following phrase:
at least once a year
Many protestants take that to a whole new level, and turn it completely on it's head; if you're only going to church once or twice a year, something is seriously wrong. You should be at worship services more often than not. Protestants don't need to require attendance at this time, because members attend regularly anyway. Even when travelling, many will find a local place of worship and attend with the local congregation.
To put it another way, regularly missing worship service can be seen as an act of profanity.
Let me explain: The basic definition of the word "holy" is something that is set apart. Worship time is set apart for God — and so qualifies as holy. The definition of profanity is to take something that is holy and to make it common. Since worship time happens often, it is easy to allow other things to get in the way. Nevertheless, that is the very definition of profanity; something that is holy was made common.
A place where you can see this often is with school schedules. Events like band/choir concerts, theatre productions, and sports schedules can conflict with worship schedules. These school events seem special, but they are still common relative to worship, in that they are not dedicated to God; allowing them to take precedence over worship is nothing less than profanity.
This principle especially includes "special" days; for a protestant to not attend on Christmas or Easter would be profanity... just as it would be to miss other occasions for less-than-holy reasons.
There is another point of confusion as well, and that is the relationship between individuals, Christ, and the Church; protestants tend to view this very different than do Catholics. Protestants often simply do not believe the church has authority over them, to require anything like attendance on specific days. Rather, we are a "nation of priests", and are responsible directly to Christ alone. The church acts more like a support group — a commanded support group, but that's still a reduced role compared to Catholicism.
Finally, I present Romans 14:5
Some consider one day more sacred than another; others consider every day alike. Everyone should be fully convinced in their own mind. (NIV)
I normally hesitate to bring Romans 14 into these discussions, but this particular example from the chapter matches so directly with the question. I think it's obvious here that both opinions on the matter are expressly permitted.